Robust fine frequency and time estimation in mobile receivers

ABSTRACT

A technique for estimating a carrier frequency offset and a timing offset in a MediaFLO™ (Forward Link Only) communication system, wherein the method comprises includes receiving Orthogonal Frequency Division Multiplexing (OFDM) symbols; interpolating pilots on odd or even symbols of the received OFDM symbols; determining a phase difference between two successive symbols using the interpolated pilots; obtaining an estimate of the carrier frequency offset and the timing offset from the determined phase difference between two successive symbols; and correcting a sampling frequency in accordance with the estimated carrier frequency offset and timing offset.

CROSS REFERENCE TO RELATED APPLICATION

This application is a reissue of U.S. Pat. No. 7,944,999 issued on May17, 2011, the contents of which in its entirety, is herein incorporatedby reference.

BACKGROUND

1. Technical Field

The embodiments herein generally relate to wireless communications, and,more particularly, to a method and apparatus for carrier frequencyoffset and timing offset estimation in a MediaFLO™ (Forward Link Only)system.

2. Description of the Related Art

In recent years, the wireless industry has seen explosive growth indevice capability, especially in relation to mobile devices, such ascell phones, handhelds, gaming consoles, etc. Ever-increasing demand forcomputing power, memory, and high-end graphic functionalities hasaccelerated the development of new and exciting wireless services. Inthe last few years, multiple technologies have been proposed to addressdelivery of streaming multimedia to mobile devices.

Multimedia communications provide a rich and immediate environment ofimage, graphics, sound, text and interaction through a range oftechnologies. An example of multimedia communication is streamingmultimedia which is primarily a delivery of continuous synchronizedmedia data. The streaming multimedia is constantly received by, anddisplayed to an end user while it is being delivered by a provider.Multiple technologies such as Integrated Services DigitalBroadcasting-Terrestrial (ISDB-T), Terrestrial-Digital MultimediaBroadcasting (T-DMB), Satellite-Digital Multimedia Broadcasting (S-DMB),Digital Video Broadcasting-Handheld (DVB-H), and FLO (Forward Link Only)are used to address the delivery of streaming multimedia to mobiledevices. These technologies have typically leveraged upon either thirdgeneration cellular/PCS, or digital terrestrial TV broadcasttechnologies.

For delivering unprecedented volumes of high-quality, streaming orclipped, audio and video multimedia to wireless subscribers, an airinterface has been developed based on FLO technology for MediaFLO™mobile multimedia multicast system available from Qualcomm, Inc.,California, USA. MediaFLO™ or media forward link only is a combinationof the media distribution system and the FLO technology. The FLOtechnology is the ability to deliver a rich variety of content choice toconsumers while efficiently utilizing spectrum as well as effectivelymanaging capital and operating expenses for service providers. Thedetails of the MediaFLO™ mobile multimedia multicast system areavailable in Chari, M. et al., “FLO Physical Layer: An Overview,” IEEETransactions on Broadcasting, Vol. 53, No. 1, March 2007, the contentsof which, in its entirety, is herein incorporated by reference.

FLO technology was designed specifically for the efficient andeconomical distribution of the same multimedia content to millions ofwireless subscribers simultaneously. Also, the FLO technology wasdesigned from the ground up to be a multicasting network, which isoverlaid upon a cellular network. It does not need to support anybackward compatibility constraints. Thus, both the networkinfrastructure and the receiver devices are separate from those for thecellular/PCS network. Moreover, as the name suggests, the technologyrelies on the use of a forward link (network to device) only.

FLO enables reducing the cost of delivering such content and enhancingthe user experience, allowing consumers to “surf” channels of content onthe same mobile handsets they use for traditional cellular voice anddata services. MediaFLO™ technology can provide robust mobileperformance and high capacity without compromising power consumption.The technology also reduces the network cost of delivering multimediacontent by dramatically decreasing the number of transmitters needed tobe deployed. In addition, MediaFLO™ technology-based multimediamulticasting complements wireless operators' cellular network data andvoice services, delivering content to the same cellular handsets used on3G networks.

The MediaFLO™ wireless system has been designed to broadcast real timeaudio and video signals, apart from non-real time services to mobileusers. The system complements existing networks and radically expandsthe ability to deliver desired content without impacting the voice anddata services. Operators can leverage the MediaFLO™ system to increaseaverage revenue per user (ARPU) and reduce churn by offering enhancedmultimedia services. Content providers can take advantage of a newdistribution channel to extend their brand to mobile users. Devicemanufacturers will benefit from increased demand for multimedia-enabledhandsets as consumer appetite grows for the rich content providedthrough MediaFLO™ systems.

The MediaFLO™ service is designed to provide the user with a viewingexperience similar to a television viewing experience by providing afamiliar type of program-guide user interface. Users can simply select apresentation package, or grouping of programs, just as they would selecta channel to subscribe to on television. Once the programs are selectedand subscribed to, the user can view the available programming contentat any time. In addition to viewing high quality video and audio contentand IP data, the user may also have access to related interactiveservices, including the option to purchase a music album, ring tone, ordownload of a song featured in a music program. The user can alsopurchase access to on-demand video programming, above and beyond thecontent featured on the program guide.

The respective MediaFLO™ system transmission is carried out using talland high power transmitters to ensure wide coverage in a givengeographical area. Further, it is common to deploy 3-4 transmitters inmost markets to ensure that the MediaFLO™ system signal reaches asignificant portion of the population in a given market. During theacquisition process of a MediaFLO™ system data packet severaldeterminations and computations are made to determine such aspects asfrequency offsets for the respective wireless receiver. Given the natureof MediaFLO™ system broadcasts that support multimedia dataacquisitions, efficient processing of such data and associated overheadinformation is paramount.

For instance, in a typical communication receiver design, the samplingtime of the receiver is usually not commensurate with that of thetransmitter, and a carrier and time offset exists between thetransmitter and the receiver. Therefore, the resulting carrier and timeoffset need to be estimated and then corrected to ensure reliablequality communication. Similarly, in Orthogonal Frequency DivisionMultiplexing (OFDM) based communication systems like MediaFLO™ formobile TV broadcasting applications, the phase difference betweensuccessive OFDM symbols are first taken on corresponding pilots toobtain a fine estimate of the carrier and time offset, and the channelfrequency response are then estimated and equalized. However, inMediaFLO™ receiver design, since the OFDM pilots exists every other OFDMsymbols, the carrier offset estimation range are limited to +/−0.25 OFDMsub carrier spacing, which corresponds to about +/−340 Hz carrierfrequency offset. In practice this range might not be sufficient.Accordingly, there remains a need for a novel Doppler frequencyestimation technique that permits estimation of high Doppler frequenciesto further increase the fine carrier frequency estimation range inMediaFLO™ receiver design to about +/−0.5 OFDM sub carrier spacing,which corresponds to about +/−680 Hz at almost no complexity increment.

SUMMARY

In view of the foregoing, an embodiment herein provides a method ofestimating a carrier frequency offset and a timing offset in a MediaFLO™(Forward Link Only) system, and a program storage device readable bycomputer, tangibly embodying a program of instructions executable bysaid computer to perform the method of estimating a carrier frequencyoffset and a timing offset in a MediaFLO™ (Forward Link Only) system,wherein the method comprises receiving OFDM symbols; interpolatingpilots on odd or even symbols of the received OFDM symbols; determininga phase difference between two successive symbols using the interpolatedpilots; obtaining an estimate of the carrier frequency offset and thetiming offset from the determined phase difference between twosuccessive symbols; and correcting a sampling frequency in accordancewith the estimated carrier frequency offset and timing offset.

Preferably, determining the phase difference occurs using relation:

${{\Delta\varphi}_{k} = {2{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein Δφ_(k) is a differential phase between two successive symbols ofsub-carrier index k in rad/symbol, Δf is the carrier offset between areceiver and a transmitter in said MediaFLO™ (Forward Link Only)receiver system in terms of sub-carrier bin duration,

${\delta = \frac{T - T^{\prime}}{T^{\prime}}},$where T is a transmitter sampling period and T′ is a receiver samplingperiod, k is the sub-carrier index, and T_(u) is an OFDM symbol durationexcluding a guard interval.

Moreover, the method may further comprise determining Δφ_(k) formultiple sub-carrier index k using said relation; and representing theresulting values of Δφ_(k) graphically. Additionally, obtaining of theestimate of the carrier frequency offset may be derived as the mean ofintercept of the graphically represented values of Δφ_(k) and the timingoffset may be derived as the slope of the graphically represented valuesof Δφ_(k).

Preferably, an estimate of the timing offset φ_(Δ) and the carrierfrequency offset φ_(μ) is obtained using:

${\varphi_{\Delta} = {\frac{4}{L^{2}}{\underset{k = 0}{\sum\limits^{\frac{L}{2} - 1}}\left( {{\Delta\varphi}_{\frac{L}{2} + k} - {\Delta\varphi}_{k}} \right)}}},{\varphi_{u} = {\frac{1}{L}{\underset{k = 0}{\sum\limits^{L - 1}}{\Delta\varphi}_{k}}}}$wherein L is a total number pilots involved in the estimation within oneOFDM symbol.

Furthermore, a relationship between the phase difference Δφ_(k), thetiming offset δ, and the carrier offset Δf is given by:

${{\Delta\varphi}_{k} = {4{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein when there is no timing offset, Δf takes a maximum value whenΔφ_(k)=±π.

Another embodiment includes an apparatus for estimating a carrierfrequency offset and a timing offset in a MediaFLO™ (Forward Link Only)system, wherein the apparatus comprises a receiver adapted to receiveOFDM symbols; a processor adapted to interpolate pilots on odd or evensymbols of the received OFDM symbols; a calculator adapted to determinea phase difference between two successive symbols using the interpolatedpilots; an estimator adapted to obtain an estimate of the carrierfrequency offset and the timing offset from the determined phasedifference between two successive symbols; and means for correcting asampling frequency in accordance with the estimated carrier frequencyoffset and timing offset.

The calculator may be further adapted to determine the phase differenceusing relation:

${{\Delta\varphi}_{k} = {2{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein Δφ_(k) is a differential phase between two successive symbols ofsub-carrier index k in rad/symbol, Δf is the carrier offset between areceiver and a transmitter in said MediaFLO™ (Forward Link Only)receiver system in terms of sub-carrier bin duration,

${\delta = \frac{T - T^{\prime}}{T^{\prime}}},$where T is a transmitter sampling period and T′ is a receiver samplingperiod, k is the sub-carrier index, and T_(u) is an OFDM symbol durationexcluding a guard interval.

The calculator may be further adapted to determine Δφ_(k) for multiplesub-carrier index k using said relation; and represent the resultingvalues of Δφ_(k) graphically. Furthermore, the estimate of the carrierfrequency offset may be derived as the mean of intercept of thegraphically represented values of Δφ_(k) and the timing offset may bederived as the slope of the graphically represented values of Δφ_(k).

Preferably, an estimate of the timing offset φ_(Δ) and the carrierfrequency offset φ_(μ) is obtained using:

${\varphi_{\Delta} = {\frac{4}{L^{2}}{\overset{\frac{L}{2} - 1}{\sum\limits_{k = 0}}\left( {{\Delta\varphi}_{\frac{L}{2} + k} - {\Delta\varphi}_{k}} \right)}}},{\varphi_{\mu} = {\frac{1}{L}{\overset{L - 1}{\sum\limits_{k = 0}}{\Delta\varphi}_{k}}}}$wherein L is a total number pilots involved in the estimation within oneOFDM symbol.

Furthermore, a relationship between the phase difference Δφ_(k), thetiming offset δ, and the carrier offset Δf is given by:

${{\Delta\varphi}_{k} = {4{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein when there is no timing offset, Δf takes a maximum value whenΔφ_(k)=±π.

These and other aspects of the embodiments herein will be betterappreciated and understood when considered in conjunction with thefollowing description and the accompanying drawings. It should beunderstood, however, that the following descriptions, while indicatingpreferred embodiments and numerous specific details thereof, are givenby way of illustration and not of limitation. Many changes andmodifications may be made within the scope of the embodiments hereinwithout departing from the spirit thereof, and the embodiments hereininclude all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein will be better understood from the followingdetailed description with reference to the drawings, in which:

FIG. 1 illustrates a FLO system for a MediaFLO™ system according to anembodiment herein;

FIG. 2 is a graphical representation of the relationship between thedifferential phase and frequency and timing offset;

FIG. 3 illustrates the structure of scattered OFDM pilots considered inorder to obtain an estimate of a straight line in a MediaFLO™ receiverdesign;

FIGS. 4A and 4B illustrate examples of Doppler fading effect on timingand carrier offset estimation;

FIG. 5 illustrates the structure of interpolated scattered pilots for aMediaFLO™ receiver design;

FIG. 6A illustrates a simulation result of a straight line obtainedusing a technique based on raw pilots;

FIG. 6B illustrates a simulation result of a straight line obtainedusing a technique based on interpolated pilots;

FIG. 7 is a block diagram illustrating an embodiment of a system forestimating a slope and a mean of a straight line obtained using atechnique based on interpolated pilots;

FIG. 8 is a flow diagram illustrating a preferred method according to anembodiment herein; and

FIG. 9 illustrates a schematic diagram of a computer architecture usedin accordance with the embodiments herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The embodiments herein and the various features and advantageous detailsthereof are explained more fully with reference to the non-limitingembodiments that are illustrated in the accompanying drawings anddetailed in the following description. Descriptions of well-knowncomponents and processing techniques are omitted so as to notunnecessarily obscure the embodiments herein. The examples used hereinare intended merely to facilitate an understanding of ways in which theembodiments herein may be practiced and to further enable those of skillin the art to practice the embodiments herein. Accordingly, the examplesshould not be construed as limiting the scope of the embodiments herein.

As mentioned, there remains a need to increase the fine carrierfrequency estimation range in MediaFLO™ system receiver design. Theembodiments herein achieve this by providing a technique of carrierfrequency estimation and timing estimation using interpolated pilots.Referring now to the drawings, and more particularly to FIGS. 1 through8, where similar reference characters denote corresponding featuresconsistently throughout the figures, there are shown preferredembodiments.

The FLO system is comprised of two parts: (a) The FLO network, whichincludes the collection of transmitters and the backhaul network, and(b) The FLO device, which may be any type of communicating devices suchas a cell phone, computer, personal assistant, laptop, handheld, orgaming consoles, etc. FIG. 1 illustrates a FLO system 100 for aMediaFLO™ system. The system 100 includes one or more transmitters 110that communicate across a wireless network 130 to one or more receivers120. A processor 125 receives and processes the symbol streams receivedby the receiver 120.

The FLO system 100 is utilized in developing an air interface for theMediaFLO™ mobile multicast system. The air interface uses OrthogonalFrequency Division Multiplexing (OFDM) as the modulation technique,which is also utilized by Digital Audio Broadcasting (DAB), (DVD-T), and(ISDB-T). To ensure that the user experience is as uniform as possibleover the entire coverage area and optimize spectral efficiency andnetwork economics, FLO system 100 employs the concept of SingleFrequency Network (SFN) operation.

The FLO system 100 multicasts several services. A service is anaggregation of one or more related data components, such as the video,audio, text or signaling associated with a service. In an embodiment,the services are classified into two types based on their coverage area:Wide-area services and Local-area services. A Local-area service ismulticast for reception within a metropolitan area. By contrast,Wide-area services are multicast in one or more metropolitan areas. Theterm Local-area is used to denote the transmitters within a metropolitanarea. The term Wide-area is used to denote transmitters in one or moremetropolitan areas that multicast the same Wide-area services. Thus, aWide-area contains one or more Local-areas, with the transmitters in thedifferent Local-areas multicasting different local area services and inan embodiment, using different radio frequency (RF) center frequencies.

FLO services are carried over one or more logical channels. Theselogical channels are called Multicast Logical Channels (MLC). Animportant aspect is that MLCs are distinguishable at the physical layer.For example, the video and audio components of a given service can besent on two different MLCs. A FLO device (a receiver from the pluralityof receivers 120) that is interested in the audio component can onlyreceive the corresponding MLC without receiving the MLC for the videocomponent, thereby saving battery resources.

The statistical multiplexing of different services, or MLCs, is achievedby varying only the MLC time and frequency allocations over prescribedtime intervals to match the variability in the MLC's source rates.Statistical multiplexing in FLO enables the receivers 120 to demodulateand decode only the MLC(s) of interest. The data rates required by theservices are expected to vary over a wide range, depending on theirmultimedia content. Thus, effective use of statistical multiplexing cansignificantly increase the number of services supported by a multicastsystem using a specified channel bandwidth.

In a typical OFDM based system, the sampling time of a receiver T′ isusually not commensurate with that of a transmitter T; i.e., T≠T′. Acarrier offset Δf exists between the receiver and the transmitter. Thephase difference between two successive OFDM symbols (assuming thecorresponding OFDM sub-carriers do not carry information like the pilotscarriers), then it can be shown that the following equation holds:

$\begin{matrix}{{\Delta\varphi}_{k} = {2{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}} & (1)\end{matrix}$where Δφ_(k) is the differential phase between two successive symbols ofsub-carrier index k in rad/symbol; Δf is the carrier offset between thereceiver and the transmitter in terms of sub-carrier bin duration.

${\delta = \frac{T - T^{\prime}}{T^{\prime}}},$where T is the transmitter sampling period and T′ is the receiversampling period; k is the sub-carrier index; T_(u) is the OFDM symbolduration excluding the guard interval.

FIG. 2 is a graphical representation of equation (1). As illustrated inFIG. 2, equation (1) is shown as a straight line 210, and the slope ofthe straight line 210 equals the timing offset φ_(Δ) apart from thescaling factor T_(u). The mean intercept of the straight line 210 equalsthe carrier frequency offset φ_(μ). Thus, the timing offset φ_(Δ) andthe carrier frequency offset φ_(μ) may be obtained as follows:φ_(Δ)=slope(Δφ_(k))=δ/T_(u)φ_(μ)=E[Δφ_(k)]=Δf  (2)

FIG. 3 illustrates the structure of scattered OFDM pilots considered inorder to obtain an estimate of a straight line in a MediaFLO™ systemreceiver design according to an embodiment herein. As illustrated, thephase difference across OFDM pilots are taken every other symbols; i.e.,either odd symbols 310 or even symbols 320. An estimate of the timingoffset φ_(Δ) and the carrier frequency offset φ_(μ) may then be obtainedas follows:

$\begin{matrix}{{\varphi_{\Delta} = {\frac{4}{L^{2}}{\overset{\frac{L}{2} - 1}{\sum\limits_{k = 0}}\left( {{\Delta\varphi}_{\frac{L}{2} + k} - {\Delta\varphi}_{k}} \right)}}},} & (2) \\{\varphi_{\mu} = {\frac{1}{L}{\overset{L - 1}{\sum\limits_{k = 0}}{\Delta\varphi}_{k}}}} & \;\end{matrix}$where L is the total number pilots involved in the estimation within oneOFDM symbol.

In the case of channels with high Doppler frequency, the straight line210 in FIG. 2 will not be clean and can be very noisy which leads to thewrap up of some values of Δφ_(k). Wrapping up happens for angle valuesthat exceed 2π. This is due to the fact that exp(φ+2π) is equal toexp(φ). Thus, values for angle values that exceed 2π may not bedistinguishable.

FIGS. 4A and 4B illustrate examples of Doppler fading effect on timingand carrier offset estimation. In the example of FIG. 4A, the Dopplerfading effect shown is of a TU6 channel with Doppler 150 Hz, timingoffset 100 ppm, and delta angles even index 24. In the example of FIG.4B, the Doppler fading effect shown is of a ideal channel with timingoffset 100 ppm, and delta angles even index 34. The line 410 of FIG. 4Aand the line 420 of FIG. 4B are not clean and may be very noisy, whichleads to the wrap up of some values of Δφ_(k). Thus, it is desirable touse moving averaging based techniques to smooth out the noisy effectcaused by fast Doppler channel changes. In an embodiment, a movingaveraging technique such as a leaky integrator may be used to smooth outthe noisy effect.

As the differential phase are obtained between odd symbols 310 or evensymbols 320 in FIG. 3, the relationship between the differential phaseΔφ_(k), the timing offset δ, and the carrier offset Δf for the MediaFLO™system application may be then given by the following equation:

$\begin{matrix}{{\Delta\varphi}_{k} = {4{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}} & (4)\end{matrix}$

From equation (4) it may be observed that, in case the timing offset δ=0ppm, (i.e., no timing offset) in an embodiment, Δf takes the maximumvalue when Δφ_(k)=±π, according to equation (4), Δf_(max)=0.25 (bins),which translates to approximately 340 Hz. In case the timing offset δ≠0ppm in another embodiment, it may be observed that Δf_(max)<0.25 (bins).

As it is desired to further increase the carrier and timing offsetcarrier ranges, accordingly in accordance with an embodiment, the pilotsare interpolated on the odd symbols 310 or the even symbols 320 of FIG.3. FIG. 5 illustrates the structure of interpolated scattered pilots fora MediaFLO™ system receiver design according to an embodiment herein.The interpolated pilots are then used to determine the phase differencebetween two successive symbols instead of using the raw pilots overevery other symbol. The corresponding differential phase may be thengiven by:

$\begin{matrix}{{\Delta\varphi}_{k} = {2{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}} & (5)\end{matrix}$

From equation (5), it may be observed that, in case δ=0 ppm, in anembodiment, Δf takes the maximum value when Δφ_(k)=±π, i.e.,Δf_(max)=0.5 (bins), which translates to approximately 680 Hz. Thus, theestimation ranger has been doubled using this technique.

FIG. 6A illustrates a simulation result of the straight line 610obtained using a technique based on raw pilots. As illustrated in theexample of FIG. 6A, the carrier offset tolerance is about 0.10 bin with50 ppm timing offset. Further, the straight line 610 tends to wrap withan increase of the carrier offset tolerance from 0.10 bin. FIG. 6Billustrates a simulation result of the straight line 620 obtained usinga technique based on interpolated pilots. In the example of FIG. 6B, itcan be seen that with 50 ppm timing offset, the carrier offset toleranceis about 0.25 bin. The straight line 620 tends to wrap with furtherincrease of the carrier offset tolerance from 0.25 bin. Thus, thetechnique using interpolated pilots doubles the estimation range of thecarrier offset frequency to +/−0.5 OFDM sub carrier spacing, whichcorresponds to about +/−680 Hz at almost no complexity increment.

FIG. 7 is a block diagram illustrating an embodiment of a system 700 forestimating a slope and a mean of a straight line obtained using atechnique based on interpolated pilots. The system 700 comprises angleestimators 710, 720 to estimate angles of the interpolated pilots. Theangle estimator 710 estimates an angle of current pilots and the angleestimator 720 estimates an angle of the previous pilots. A calculator730 estimates the phase difference Δφ between the current pilot and theprevious pilot. The resulting output of the calculator 730 is thenprovided to a slope estimator 740 and a mean estimator 750. Asmentioned, graphically Δφ is a straight line. The slope estimator 740estimates the slope and the mean estimator 750 estimates the meanintercept of the straight line of Δφ. To smooth out the noisy effectcaused by fast Doppler channel changes, the resulting output of theslope estimator 740 is provided to a line gradient leaky integrator 760and the resulting output of the mean estimator 750 is provided to a linemean leaky integrator 770. The resulting output of the line gradientleaky integrator 760 provides the slope of the straight line of Δφ,wherein the slope equals the timing offset φ_(Δ) and the resultingoutput of the line mean leaky integrator 770 provides the mean interceptof the straight line of Δφ, wherein the mean intercept equals thefrequency offset φ_(μ).

FIG. 8, with reference to FIGS. 1 through 7, illustrates a flow diagramillustrating a method of estimating a carrier frequency offset and atiming offset in a MediaFLO™ (Forward Link Only) system according to anembodiment herein, wherein the method comprises receiving (801)Orthogonal Frequency Division Multiplexing (OFDM) symbols; interpolating(803) pilots on odd or even symbols of the received OFDM symbols;determining (805) a phase difference between two successive symbolsusing the interpolated pilots; obtaining (807) an estimate of thecarrier frequency offset and the timing offset from the determined phasedifference between two successive symbols; and correcting (809) asampling frequency in accordance with the estimated carrier frequencyoffset and timing offset.

Preferably, determining (805) the phase difference occurs usingrelation:

${{\Delta\varphi}_{k} = {2{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein Δφ_(k) is a differential phase between two successive symbols ofsub-carrier index k in rad/symbol, Δf is the carrier offset between areceiver and a transmitter in said MediaFLO™ (Forward Link Only)receiver system in terms of sub-carrier bin duration,

${\delta = \frac{T - T^{\prime}}{T^{\prime}}},$where T is a transmitter sampling period and T′ is a receiver samplingperiod, k is the sub-carrier index, and T_(u) is an OFDM symbol durationexcluding a guard interval.

Moreover, the method may further comprise determining Δφ_(k) formultiple sub-carrier index k using said relation; and representing theresulting values of Δφ_(k) graphically. Additionally, obtaining (807) ofthe estimate of the carrier frequency offset may be derived as the meanof intercept of the graphically represented values of Δφ_(k) and thetiming offset may be derived as the slope of the graphically representedvalues of Δφ_(k).

Preferably, an estimate of the timing offset φ_(Δ) and the carrierfrequency offset φ_(μ) is obtained using:

$\begin{matrix}{{\varphi_{\Delta} = {\frac{4}{L^{2}}{\overset{\frac{L}{2} - 1}{\sum\limits_{k = 0}}\left( {{\Delta\varphi}_{\frac{L}{2} + k} - {\Delta\varphi}_{k}} \right)}}},} \\{\varphi_{\mu} = {\frac{1}{L}{\overset{L - 1}{\sum\limits_{k = 0}}{\Delta\varphi}_{k}}}}\end{matrix}$wherein L is a total number pilots involved in the estimation within oneOFDM symbol.

Further, a relationship between the phase difference Δφ_(k), the timingoffset δ, and the carrier offset Δf is given by:

${{\Delta\varphi}_{k} = {4{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein when there is no timing offset, Δf takes a maximum value whenΔφ_(k)=±π.

The techniques provided by the embodiments herein may be implemented onan integrated circuit chip (not shown). The chip design is created in agraphical computer programming language, and stored in a computerstorage medium (such as a disk, tape, physical hard drive, or virtualhard drive such as in a storage access network). If the designer doesnot fabricate chips or the photolithographic masks used to fabricatechips, the designer transmits the resulting design by physical means(e.g., by providing a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design is then converted into the appropriateformat (e.g., GDSII) for the fabrication of photolithographic masks,which typically include multiple copies of the chip design in questionthat are to be formed on a wafer. The photolithographic masks areutilized to define areas of the wafer (and/or the layers thereon) to beetched or otherwise processed.

The resulting integrated circuit chips can be distributed by thefabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

The embodiments herein can take the form of an entirely hardwareembodiment, an entirely software embodiment or an embodiment includingboth hardware and software elements. The embodiments that areimplemented in software include but are not limited to, firmware,resident software, microcode, etc.

Furthermore, the embodiments herein can take the form of a computerprogram product accessible from a computer-usable or computer-readablemedium providing program code for use by or in connection with acomputer or any instruction execution system. For the purposes of thisdescription, a computer-usable or computer readable medium can be anyapparatus that can comprise, store, communicate, propagate, or transportthe program for use by or in connection with the instruction executionsystem, apparatus, or device.

The medium can be an electronic, magnetic, optical, electromagnetic,infrared, or semiconductor system (or apparatus or device) or apropagation medium. Examples of a computer-readable medium include asemiconductor or solid state memory, magnetic tape, a removable computerdiskette, a random access memory (RAM), a read-only memory (ROM), arigid magnetic disk and an optical disk. Current examples of opticaldisks include compact disk-read only memory (CD-ROM), compactdisk-read/write (CD-R/W) and DVD.

A data processing system suitable for storing and/or executing programcode will include at least one processor coupled directly or indirectlyto memory elements through a system bus. The memory elements can includelocal memory employed during actual execution of the program code, bulkstorage, and cache memories which provide temporary storage of at leastsome program code in order to reduce the number of times code must beretrieved from bulk storage during execution.

Input/output (I/O) devices (including but not limited to keyboards,displays, pointing devices, etc.) can be coupled to the system eitherdirectly or through intervening I/O controllers. Network adapters mayalso be coupled to the system to enable the data processing system tobecome coupled to other data processing systems or remote printers orstorage devices through intervening private or public networks. Modems,cable modem and Ethernet cards are just a few of the currently availabletypes of network adapters.

A representative hardware environment for practicing the embodimentsherein is depicted in FIG. 9. This schematic drawing illustrates ahardware configuration of an information handling/computer system 900 inaccordance with the embodiments herein. The system 900 comprises atleast one processor or central processing unit (CPU) 910. The CPUs 910are interconnected via system bus 912 to various devices such as arandom access memory (RAM) 914, read-only memory (ROM) 916, and aninput/output (I/O) adapter 918. The I/O adapter 918 can connect toperipheral devices, such as disk units 911 and tape drives 913, or otherprogram storage devices that are readable by the system 900. The system900 can read the inventive instructions on the program storage devicesand follow these instructions to execute the methodology of theembodiments herein. The system 900 further includes a user interfaceadapter 919 that connects a keyboard 915, mouse 917, speaker 924,microphone 922, and/or other user interface devices such as a touchscreen device (not shown) to the bus 912 to gather user input.Additionally, a communication adapter 920 connects the bus 912 to a dataprocessing network 925, and a display adapter 921 connects the bus 912to a display device 923 which may be embodied as an output device suchas a monitor, printer, or transmitter, for example.

The sampling time of the receiver is not commensurate with that of thetransmitter, and a carrier and time offset exists between thetransmitter and the receiver. To ensure efficient communication betweenthe transmitter and the receiver, the carrier and time offset need to beestimated and then corrected to ensure reliable quality communication.Accordingly, the embodiments herein provide a manner of achieving this.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the embodiments herein that others can, byapplying current knowledge, readily modify and/or adapt for variousapplications such specific embodiments without departing from thegeneric concept, and, therefore, such adaptations and modificationsshould and are intended to be comprehended within the meaning and rangeof equivalents of the disclosed embodiments. It is to be understood thatthe phraseology or terminology employed herein is for the purpose ofdescription and not of limitation. Therefore, while the embodimentsherein have been described in terms of preferred embodiments, thoseskilled in the art will recognize that the embodiments herein can bepracticed with modification within the spirit and scope of the appendedclaims.

What is claimed is:
 1. A method of estimating a carrier frequency offsetand a timing offset in a mobile multimedia multicast communicationsystem, said method comprising: receiving Orthogonal Frequency DivisionMultiplexing (OFDM) symbols in a receiver; interpolating pilots on oddor even symbols of the received OFDM symbols; determining a phasedifference between two successive symbols using the interpolated pilots;obtaining an estimate of the carrier frequency offset and the timingoffset from the determined phase difference between two successivesymbols; and correcting a sampling frequency in accordance with theestimated carrier frequency offset and timing offset.
 2. The method ofclaim 1, wherein determining the phase difference occurs using relation:${{\Delta\varphi}_{k} = {2{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein Δφ_(k) is a differential phase between two successive symbols ofsub-carrier index k in rad/symbol, Δf is the a carrier offset between athe receiver and a transmitter in said mobile multimedia multicastcommunication system in terms of sub-carrier bin duration,${\delta = \frac{T - T^{\prime}}{T^{\prime}}},$ where T is a transmittersampling period and T′ is a receiver sampling period, k is thesub-carrier index, and T_(u) is an OFDM symbol duration excluding aguard interval.
 3. The method of claim 2, further comprising:determining Δφ_(k) for multiple sub-carrier index k using said relation;and representing the resulting values of Δφ_(k) graphically.
 4. Themethod of claim 3, wherein the obtaining of the estimate of the carrierfrequency offset is derived as the mean of intercept of the graphicallyrepresented values of Δφ_(k) and the timing offset is derived as theslope of the graphically represented values of Δφ_(k).
 5. The method ofclaim 1, wherein an estimate of the timing offset φ_(Δ) and the carrierfrequency offset φ_(μ) is obtained using:${\varphi_{\Delta} = {\frac{4}{L^{2}}{\overset{\frac{L}{2} - 1}{\sum\limits_{k = 0}}\left( {{\Delta\varphi}_{\frac{L}{2} + k} - {\Delta\varphi}_{k}} \right)}}},{\varphi_{\mu} = {\frac{1}{L}{\underset{k = 0}{\sum\limits^{L - 1}}{\Delta\varphi}_{k}}}}$wherein L is a total number pilots involved in the estimation within oneOFDM symbol, wherein ${\Delta\varphi}_{\frac{L}{2} + k}$ is a phasedifference between a current pilot and a previous pilot defined by halfof said total number of pilots plus a value of a sub-carrier index k inrad/symbol; and wherein Δφ_(k) is a differential phase between twosuccessive symbols of sub-carrier index k in rad/symbol.
 6. The methodof claim 1, wherein a relationship between the phase difference Δφ_(k),the timing offset represented by δ, and the a carrier offset representedby Δf is given by:${{\Delta\varphi}_{k} = {4{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein when there is no timing offset, Δf takes a maximum value whenΔφ_(k)=±π, and wherein k is the sub-carrier index, and T_(u) is an OFDMsymbol duration excluding a guard interval.
 7. A non-transitory programstorage device readable by computer, tangibly embodying a program ofinstructions executable by said computer to perform a method ofestimating a carrier frequency offset and a timing offset in a mobilemultimedia multicast communication system, said method comprising:receiving Orthogonal Frequency Division Multiplexing (OFDM) symbols in areceiver; interpolating pilots on odd or even symbols of the receivedOFDM symbols; determining a phase difference between two successivesymbols using the interpolated pilots; obtaining an estimate of thecarrier frequency offset and the timing offset from the determined phasedifference between two successive symbols; and correcting a samplingfrequency in accordance with the estimated carrier frequency offset andtiming offset.
 8. The program storage device of claim 7, whereindetermining the phase difference occurs using relation:${{\Delta\varphi}_{k} = {2{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein Δφ_(k) is a differential phase between two successive symbols ofsub-carrier index k in rad/symbol, Δf is the a carrier offset between athe receiver and a transmitter in said mobile multimedia multicastcommunication system in terms of sub-carrier bin duration,${\delta = \frac{T - T^{\prime}}{T^{\prime}}},$ where T is a transmittersampling period and T′ is a receiver sampling period, k is thesub-carrier index, and T_(u) is an OFDM symbol duration excluding aguard interval.
 9. The program storage device of claim 8, wherein saidmethod further comprises: determining Δφ_(k) for multiple sub-carrierindex k using said relation; and representing the resulting values ofΔφ_(k) graphically.
 10. The program storage device of claim 9, whereinthe obtaining of the estimate of the carrier frequency offset is derivedas the mean of intercept of the graphically represented values of Δφ_(k)and the timing offset is derived as the slope of the graphicallyrepresented values of Δφ_(k).
 11. The program storage device of claim 7,wherein an estimate of the timing offset φ_(Δ) and the carrier frequencyoffset φ_(μ) is obtained using:${\varphi_{\Delta} = {\frac{4}{L^{2}}{\overset{\frac{L}{2} - 1}{\sum\limits_{k = 0}}\left( {{\Delta\varphi}_{\frac{L}{2} + k} - {\Delta\varphi}_{k}} \right)}}},{\varphi_{\mu} = {\frac{1}{L}{\underset{k = 0}{\sum\limits^{L - 1}}{\Delta\varphi}_{k}}}}$wherein L is a total number pilots involved in the estimation within oneOFDM symbol, wherein ${\Delta\varphi}_{\frac{L}{2} + k}$ is a phasebetween a current pilot and a previous pilot defined by half of saidtotal number of pilots plus a value of a sub-carrier index k inrad/symbol; and wherein Δφ_(k) is a differential phase between twosuccessive symbols of sub-carrier index k in rad/symbol.
 12. The programstorage device of claim 7, wherein a relationship between the phasedifference Δφ_(k), the timing offset represented by δ, and the a carrieroffset represented by Δf is given by:${{\Delta\varphi}_{k} = {4{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein when there is no timing offset, Δf takes a maximum value whenΔφ_(k)=±π, and wherein k is the sub-carrier index, and T_(u) is an OFDMsymbol duration excluding a guard interval.
 13. An apparatus forestimating a carrier frequency offset and a timing offset in a mobilemultimedia multicast communication system, said apparatus comprising: areceiver adapted to receive Orthogonal Frequency Division Multiplexing(OFDM) symbols; a processor adapted to interpolate pilots on odd or evensymbols of the received OFDM symbols; a calculator adapted to determinea phase difference between two successive symbols using the interpolatedpilots; an estimator adapted to obtain an estimate of the carrierfrequency offset and the timing offset from the determined phasedifference between two successive symbols; and an integrator adapted tocorrect a sampling frequency in accordance with the estimated carrierfrequency offset and timing offset.
 14. The apparatus of claim 13,wherein determining the phase difference occurs using relation:${{\Delta\varphi}_{k} = {2{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein Δφ_(k) is a differential phase between two successive symbols ofsub-carrier index k in rad/symbol, Δf is the a carrier offset betweensaid receiver and a transmitter in said mobile multimedia multicastcommunication system in terms of sub-carrier bin duration,${\delta = \frac{T - T^{\prime}}{T^{\prime}}},$ where T is a transmittersampling period and T′ is a receiver sampling period, k is thesub-carrier index, and T_(u) is an OFDM symbol duration excluding aguard interval.
 15. The apparatus of claim 14, wherein said${\Delta\varphi}_{k} = {2{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}$is determined for multiple sub-carrier index k, and wherein theresulting values of Δφ_(k) are represented graphically.
 16. Theapparatus of claim 15, wherein the obtaining of the estimate of thecarrier frequency offset is derived as the mean of intercept of thegraphically represented values of Δφ_(k) and the timing offset isderived as the slope of the graphically represented values of Δφ_(k).17. The apparatus of claim 13, wherein an estimate of the timing offsetφ_(Δ) and the carrier frequency offset φ_(μ) is obtained using:${\varphi_{\Delta} = {\frac{4}{L^{2}}{\overset{\frac{L}{2} - 1}{\sum\limits_{k = 0}}\left( {{\Delta\varphi}_{\frac{L}{2} + k} - {\Delta\varphi}_{k}} \right)}}},{\varphi_{\mu} = {\frac{1}{L}{\underset{k = 0}{\sum\limits^{L - 1}}{\Delta\varphi}_{k}}}}$wherein L is a total number pilots involved in the estimation within oneOFDM symbol, wherein ${\Delta\varphi}_{\frac{L}{2} + k}$ is a phasedifference between a current pilot and a previous pilot defined by halfof said total number of pilots plus a value of a sub-carrier index k inrad/symbol; and wherein Δφ_(k) is a differential phase between twosuccessive symbols of sub-carrier index k in rad/symbol.
 18. Theapparatus of claim 13, wherein a relationship between the phasedifference Δφ_(k), the timing offset represented by δ, and the a carrieroffset represented by Δf is given by:${{\Delta\varphi}_{k} = {4{\pi\left( {{\Delta f} + {\frac{\delta}{T_{u}} \cdot k}} \right)}}},$wherein when there is no timing offset, Δf takes a maximum value whenΔφ_(k)=±π, and wherein k is the sub-carrier index, and T_(u) is an OFDMsymbol duration excluding a guard interval.
 19. The apparatus of claim13, further comprising a transmitter adapted to transmit said OFDMsymbols.
 20. The apparatus of claim 19, further comprising acommunication link between said receiver and said transmitter.
 21. Themethod of claim 1, wherein said communication system comprises a mobiletelevision communication system.
 22. The program storage device of claim7, wherein said communication system comprises a mobile televisioncommunication system.
 23. The apparatus of claim 13, wherein saidcommunication system comprises a mobile television communication system.